31.4 - Virtual and real images

Image: A reproduction of an

object by means of light.

Virtual image: An image on

the opposite side of a mirror

than the object it is created

from. The image cannot be

projected onto paper at its

perceived location.

Real image: An image on the same side of a

mirror as the object it is created from. The

image can be projected onto paper at its

perceived location.

When you look into a planar mirror, your image appears to be on the opposite side of

the mirror, in a “looking glass” world that you cannot reach except in literature. The light

of the image registered by your eyes is not actually coming from behind the mirror, even

though your brain may interpret it this way.

You may be so accustomed to virtual images such as those formed by planar mirrors

that you take them for granted and believe them to be as “real” as any other image. But

not as real as any other object: A true understanding of virtual images must be learned.

A kitten, when first exposed to a mirror, believes its reflected image to be another kitten

and will search for the potential playmate behind the mirror. Over time, some animals

(including humans, chimpanzees and dolphins) can learn how to interpret the nature of

the images created by mirrors.

It will now be helpful to be more formal about some basic terminology. Objects are “in

front” of a mirror: They are on its reflecting side. (In the diagrams in this book we will

almost always place an object to the left of the mirror, but that certainly is not a law of

physics, just a convention of sorts about which side is “in front.”) If the image is on the

same side of the mirror as the object, it also is in front of the mirror. If the image is on

the opposite side of the mirror from the object, we say it is “behind” or in back of the

mirror. A virtual image created by a mirror is behind the mirror. The distance between a

virtual image and the mirror is negative.

Curved mirrors can create what is called a real image. In Concept 3, you see a diagram

of a concave mirror creating a real image: an image on the same side of the mirror as

the object. By convention, the distance between the mirror and the real image is

positive. We stress the conventions of sign because they will prove essential when

equations are used to calculate certain properties of an image, such as its position.

The terms “real” and “virtual” can be confusing. One way to decide whether an image is

real or virtual is to imagine placing a piece of paper at the location of the image and

then observing the results. With real images, the light passes through the location of the

image; with virtual images, it does not. If you place a piece of paper “behind” a planar

mirror, where the virtual image is located, you will not find an image projected there.

With virtual images the brain projects the location of the image as it interprets the light it

receives.

On the other hand, if you place a piece of paper at the location of a real image, you will

see the image on the paper. For instance, images created by movie projectors are real,

which is why you can see them on movie screens.

Concept 4 shows an optical illusion created by a popular toy. A special combination of

curved mirrors creates a real image that is so solid looking you are tempted to reach out

and grasp it.

Kitten looking for its image behind a mirror.

Image

A reproduction created by light

Virtual image

Opposite side of mirror

Negative distance

Real image

Same side of mirror

Positive distance

(^576) Copyright 2007 Kinetic Books Co. Chapter 31